Advanced digital flux gate magnetometer

ABSTRACT

Digital logic comprising a high speed analog to digital converter, a high speed analog to digital converter, a digital multiplier, and an infinite impulse response filter and decimation circuit are used to digitize a magnetic signal that is subsequently processed by magnetic data processing algorithms. The high resolution digital to analog converter is directly incorporated in a feedback loop of a magnetometer sensor, and an expensive precision analog to digital converter is replaced by a less expensive, lower power digital to analog converter. The improvement provided by the present invention includes a high speed analog to digital converter coupled between the oscillator and a first input of a multiplier, and a second analog to digital converter that is coupled between the sense coil of the sensor and a second input of the multiplier. The output of the multiplier is coupled to an infinite impulse response filter and decimation circuit that produces the digital output signal from the magnetometer. The output of the infinite impulse response filter and decimation circuit is coupled by way of a digital to analog converter to the feedback coil of the sensor. Depending on the system requirements, the two analog to digital converters, the digital multiplier, and the infinite impulse response filter and decimation circuit may be implemented in one readily available microcontroller at low cost. This serves to reduce the number of components, complexity, and expense of the magnetometer.

BACKGROUND

The present invention relates generally to magnetometers, and more particularly, to a digital flux gate magnetometer.

The assignee of the present invention uses data from three axis flux gate magnetometers for its magnetic data processing systems such as those described in U.S. Pat. No. 5,239,474, and U.S. patent application Ser. No. 08/611,352, filed Mar. 5, 1996. The present state of the art employs an analog magnetic flux gate sensor and uses the output of an analog integrator as the magnetometer output. This signal is then digitized with an analog to digital converter having a large number of bits of resolution (typically >20). This requires the use of large, expensive, and power hungry analog to digital converters that presently have a dynamic range that is limited to around 22 bits of resolution. The analog integrator used in the analog flux gate sensor causes drift in the output signal of the sensor. The output of the analog flux gate sensor varies with temperature due to the non-constant resistance of the core windings with temperature.

A stopgap measure presently being utilized to get around the use of the expensive analog to digital converter is to employ an adjustable voltage reference and an analog subtractor to subtract the large constant part of the measured magnetic field due to the earth's magnetic field. Then, the remaining signal is digitized with a reduced resolution analog to digital converter (12 to 16 bits). This technique only works for applications in which the sensor is stationary with respect to the earth's magnetic field.

Alternatively, prior art magnetometers have used a very low frequency high pass filter to remove the constant component of the local field and then the remaining signal is digitized by an analog to digital converter. The prior art systems have the following drawbacks: They exhibit loss of absolute magnetic field values (which are useful for determining the heading of the sensor). They exhibit distortion (due to the high pass filtering) of the low frequency wave forms. Such distortion adversely impacts the performance the magnetic processing algorithms. They exhibit susceptibility to overload if the sensor is used in an application in which it is moved with respect to the ambient magnetic field.

Accordingly, it is an objective of the present invention to provide for a digital flux gate magnetometer that overcomes the limitations of conventional magnetometers.

SUMMARY OF THE INVENTION

To meet the above and other objectives, the present invention provides for a technique whereby digital logic and a high resolution digital to analog converter are used to adequately digitize a magnetic signal so that it may be subsequently processed by magnetic data processing systems developed by the assignee of the present invention. By incorporating a high resolution digital to analog converter directly in a feedback loop of a magnetometer sensor, an expensive precision analog to digital converter is replaced by a less expensive, lower power digital to analog converter.

More specifically the present invention provides for a flux gate magnetometer comprising an oscillator for generating a signal, a divider coupled to the oscillator for dividing the frequency of the signal by two, a sensor coupled to the divider that includes a drive coil, magnetic core material, a feedback coil, a sense coil, and a multiplier coupled to the sense coil and the oscillator. The improvement provided by the present invention includes first and second analog to digital converters that are respectively coupled between the oscillator and sense coil and a digital multiplier. The output of the multiplier is coupled to an infinite impulse response (IIR) filter and decimation circuit which produces the digital output signal from the magnetometer. The output of the infinite impulse response filter and decimation circuit is coupled by way of a digital to analog (D/A) converter to the feedback coil of the sensor.

In the present invention, the signal from the sense coil is digitized using a high speed analog to digital converter. Additionally, the signal from the oscillator is also digitized, and then the two signals are multiplied in the digital domain. The A/D converter used for the oscillator signal may be very simple, potentially only one bit. The digital product of these two signals contains a low frequency component. Because of the physics of the core material, this low frequency component of the digital product of these two signals is indicative of the low frequency magnetic field seen by the core material. During operation of the sensor, this low frequency signal is forced (via control of the feedback coil drive current) to be as close to zero as possible.

To extract and process the low frequency component, the digital product of the two inputs is filtered by the infinite impulse response filter. This provides for both integration and low pass filtering required for operation of the sensor. The digital data stream is then decimated and sent to a high resolution digital to analog converter, and to the digital output line of the magnetometer. The D/A converter drives the feedback coil so that the total low frequency magnetic field seen by the sensor core material is forced to zero. When the sensor is in equilibrium, the output of digital multiplier is a zero mean signal.

Depending on the system requirements, the two analog to digital converters, the digital multiplier, and the infinite impulse response filter may be implemented in one readily available microcontroller at low cost. This serves to reduce the number of components, complexity, and expense of the magnetometer.

In addition, the resolution of the digital to the analog to digital converter can be increased by rapidly changing the digital input applied thereto and then low pass filtering the output thereof. This approach sacrifices frequency bandwidth for increased resolution. Three or more bits may be added to the resolution of available digital to analog converters by dithering the digital input and low pass filtering the output thereof. This optional aspect of the present invention provided by a dither circuit and low pass filter before and after the digital to analog converter.

In general, magnetic flux gate sensors rely on a current feedback loop. This current is determined by taking the voltage of the circuit, and dividing it by the resistance of the (typically copper) core winding. The resistance of the core winding varies with temperature, causing the sensor measurements to drift with temperature. Commonly available high resolution D/A converters put out a current that is a function of the input signal. In the present invention, this current is constant over the varying resistance of the core winding, causing the present invention to be much less temperature dependent.

The present invention decreases power consumption by replacing a high resolution (typically >20 bit) analog to digital converter with a similarly sized digital to analog converter. This reduces the power consumption. The present invention eliminates the low frequency circuit noise and drift caused by the imperfect analog integrator that is typically used in flux gate magnetic sensors. Additionally, the superior performance available with digital signal processing techniques provides improved sensor performance with less variability due to component variations. The present invention decreases the circuit size requirements, as high resolution analog to digital converters are considerably larger than high resolution digital to analog converters.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

FIG. 1 shows a conventional flux gate magnetometer which is improved upon by the present invention; and

FIG. 2 shows a digital flux gate magnetometer in accordance with the principles of the present invention.

DETAILED DESCRIPTION

The present invention provides for improvements to current magnetic flux gate sensor technology. Referring to the drawing figures, FIG. 1 shows a conventional flux gate magnetometer 10 which is improved upon by the present invention, and which is shown for the purposes of understanding the present invention.

In the conventional flux gate magnetometer 10, an oscillator 11 generates a signal, and the frequency of this signal is divided by two in a divider 12 and is used to drive magnetic core material 15 of a magnetic sensor 13. The sensor 13 includes a drive coil 14, the magnetic core material 15, a feedback coil 16, and a sense coil 17. The core material 15 is a magnetically saturatable material that is driven into the saturated state twice with every two cycles of the waveform of the output signal from the oscillator 11.

The sense coil 17 is coupled to a first input of an analog multiplier 18 while the output signal from the oscillator 11 is coupled to a second input of the analog multiplier 18. The output of the analog multiplier 18 is integrated in an integrator 19 that provides an output signal from the magnetometer 10, which output signal is fed back to the feedback coil 16 of the sensor 13.

The nature of the core material 15 is such that if it is in a magnetic field, the sense coil 17 detects an AC signal at twice the drive frequency. This signal is multiplied (in the analog domain) with twice the drive coil frequency thereby frequency shifting the signal down to DC. This DC signal represents the difference in magnetic field seen by the core material 15 due to the ambient field (the field that is sensed) and the magnetic field due to the feedback coil 16.

Because the feedback and sense coils 16, 17 operate at different frequencies, in some sensors 13 they are the same coil appropriately coupled to the sensor 13. Thus, the integrator 19 corrects the drive to the feedback coil 16 to keep the output of the analog multiplier 18 (and therefore the magnetic field as seen by the core material 15) as close to zero as possible.

When the circuit is in equilibrium, the output of the integrator 19 is a signal equal and opposite to the magnetic field that is measured by the core material 15. The output signal of the magnetometer 10 is taken from this point. For many applications, the output signal is then digitized to a provide high resolution (often >20 bits) signal. This digitization requires an expensive analog to digital converter (not shown).

The present invention provides an improved arrangement for generating a digital output signal from the magnetic flux gate sensor 13. A block diagram of a digital flux gate magnetometer 20 in accordance with the principles of the present invention is shown in FIG. 2.

The improvement provided by the present invention includes a high speed analog to digital (A/D) converter 22 coupled between the oscillator 11 and a first input of a multiplier 24, and a second analog to digital (A/D) converter 23 that is coupled between the sense coil 17 of the sensor 13 and a second input of the multiplier 24. The output of the multiplier 24 is coupled to an infinite impulse response (IIR) filter and decimation circuit 25 that produces the digital output signal from the magnetometer 20. The output of the infinite impulse response filter and decimation circuit 25 is coupled by way of a digital to analog (D/A) converter 26 to the feedback coil 16 of the sensor 13.

In the present invention, the signal from the sense coil 17 is digitized using the high speed analog to digital converter 22. Additionally, the signal from the oscillator 11 is also digitized using the second analog to digital converter 23, and then the two signals are multiplied in the digital domain using the multiplier 24. The second analog to digital converter 23 used for the oscillator signal may be very simple, and potentially may have a resolution of only one bit. The digital product of these two signals contains a low frequency component. Because of the physics of the core material 15, this low frequency component is indicative of the low frequency magnetic field seen by the core material 15. During operation of the sensor 13, this low frequency signal is forced (via control of the drive current applied to the feedback coil 17) to be as close to zero as possible.

To extract and process the low frequency component, the digital product of the two inputs (the output of the multiplier 24) is filtered by the infinite impulse response filter of the IIR filter and decimation circuit 25. This provides for integration and low pass filtering required for operation of the sensor 13. The digital data stream is then decimated and sent to a high resolution digital to analog converter 26, and to the digital output line of the magnetometer 20. The D/A converter 26 drives the feedback coil 17 so that the total low frequency magnetic field seen by the sensor core material 15 is forced to zero. When the sensor 13 is in equilibrium, the output of digital multiplier 24 is a zero mean signal.

Depending on the system requirements, the two analog to digital converters 22, 23, the digital multiplier 24, and the infinite impulse response filter and decimation circuit 25 may be implemented in one readily available microcontroller at low cost. This serves to reduce the number of components, complexity, and expense of the magnetometer 20.

High resolution digital to analog converters 26 are readily available due to the consumer market for digital audio devices. The resolution of these high frequency devices (typically >44 KHz) can be increased by rapidly changing the digital input to the digital to the analog to digital converter 26 and then low pass filtering the output. This approach sacrifices frequency bandwidth for increased resolution. The frequencies of interest in measured magnetic fields is often one or two orders of magnitude lower than audio frequencies. Because of this, three or more bits may be added to the resolution of available consumer digital to analog converters 26 by dithering the digital input and low pass filtering the output thereof. This optional aspect of the present invention is illustrated in FIG. 2 by dashed boxes corresponding to a dither circuit 27 and low pass filter (LPF) 28 before and after the digital to analog converter 26.

Thus, an improved digital flux gate magnetometer has been disclosed. It is to be understood that the described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and varied other arrangements may be readily devised by those skilled in the art without departing from the scope of the invention. 

What is claimed is:
 1. A flux gate magnetometer comprising:an oscillator for generating a signal; a divider coupled to the oscillator for dividing the frequency of the signal by two; a sensor coupled to the divider that comprises a drive coil, magnetic core material, a feedback coil, and a sense coil; a digital multiplier; a high speed analog to digital converter coupled between the oscillator and a first input of the digital multiplier; a second analog to digital converter coupled between the sense coil of the sensor and a second input of the digital multiplier; an infinite impulse response filter and decimation circuit coupled to an output of the multiplier which produces a digital output signal from the magnetometer; and a digital to analog converter coupled between the infinite impulse response filter and decimation circuit and the feedback coil the sensor.
 2. The magnetometer of claim 1 wherein the core material is a magnetically saturatable material that is driven into the saturated state twice with every two cycles of the waveform of the output signal from the oscillator.
 3. The magnetometer of claim 1 further comprising:a dither circuit coupled between the infinite impulse response filter and decimation circuit and an input of the digital to analog converter; and a low pass filter coupled between the digital to analog converter and the sense coil of the sensor. 